In some circumstances, a particular pattern of drive activity can result in the
drive head being repeatedly parked and un-parked in short intervals, possibly*
resulting in excess wear on the drive. Apparently* the drive head parking is
recorded in the S.M.A.R.T. "Load Cycle Count" attribute.

I have two WD Red drives in my NAS, one for live data and one
for backup. The latter drive is basically unused most of the day until scheduled
backup jobs kick in and those jobs are all clustered together. I already unmount
the backup filesystems when the jobs are not active (I wrote about this in
mount-on-demand backups).

Inspecting the S.M.A.R.T. attributes was surprising:

regular

12143

348

backup

12191

13043

It certainly looks like my backup drive has a much higher load cycle count than
you might expect for a mostly-idle drive. I checked the attributes again 24 hours
later and the regular drive had incremented by a single cycle, whilst the backup
drive went up by 56.

I tried the unofficial tool to get the drive's default value, which was 0x8a,
and bump it to the maximum of 0xff. I then tried the official tool then fetched
the value again: interestingly the official tool had reset the value back to
0x8a. I haven't managed to assess the impact of these changes on the attrition
rate yet because I need to perform a cold boot for the change to take effect
and that isn't convenient just now.

My plan is to try and disable the feature completely via the unofficial tool.
If that rectifies the issue I will then investigate changing the power
management settings by hand at backup start/end time, perhaps via hdparm.

( The problem with these kind of issues is there is precious little in the way
of reliable documentation as to the real issue, real drive behaviour, etc. I've
marked a few sections of this blog post with * asterisks to indicate where we
are having to make informed guesses. )